There may be a short circuit in the switch wire, possibly caused by poor insulation.
Use a continuity tester to check if the old switch actually is faulty. If it isn't, turn off the power at the consumer unit, take the switch wires out of the ceiling rose and test the resistance between them with the switch closed, then with it open. You could even disconnect the switch to test just the wire; if there is a low resistance between the wires you may need to replace the cable.
Adam, December 2009